This phone costs about $200 new according to google search:
http://www0.shopping.com/xPO-Sony-Ericsson-Sony-Ericsson-C510-Pay-Monthly
So I'd wager if you listed this on eBay, you may be lucky to scrape around $100-120. Possibly sell for more if its unlocked and has extra accessories... so make sure you unlock it first.
Mobiles depreciate very quickly, especially when it has surface scratches.